The Mannesmann pipe making process is a widely employed process for producing seamless steel pipes by hot working. In the Mannesmann pipe making process, a round billet heated to a predetermined temperature is fed to a piercing and rolling mill (piercer) and the axial central portion of the round billet is pierced, whereby a hollow blank pipe called “hollow shell” is obtained.
The hollow shell, either as it is or after diameter expansion and wall thinning, if necessary, by passing through an elongator of the same configuration as the above-mentioned piercing and rolling mill, is fed to a following elongating rolling mill such as a plug mill or mandrel mill for elongating and rolling. Thereafter, the resulting pipe is passed through a stretch reducer, a reeler, a sizer, etc. for polishing, shape adjustment and sizing; a final product seamless steel pipe is thus produced.
In piercing and rolling by the piercing and rolling mill, a bullet-shaped plug is used as a piercing tool. This plug is mounted on the front end of a core bar or mandrel and used for piercing the billet heated to a high temperature of about 1200° C., so that it is exposed to high-temperature and high-contact-pressure conditions. Therefore, an oxide scale film is formed on the plug surface to protect the plug base metal. The scale film shields the plug against heat from the billet and at the same time plays a role in preventing seizure by the billet.
The scale film on the plug surface is generally formed by subjecting the plug made of a tool steel for how working to heat treatment at a high temperature of about 900° C. to 1000° C. for several to several tens of hours.
In recent years, the demand for seamless steel pipes has shown a trend toward a marked increase and, in particular, the increase in demand for seamless steel pipes for use in severe environments is significant. Seamless steel pipes made of such a high alloy steel species as stainless steel, Ni-based alloy steel or high-Cr-containing steel with a Cr content of 9% or higher are suited for use in severe environments.
When a scale film coated plug is used for piercing and rolling in the production of high alloy steel seamless pipes, a billet high in deformation resistance is to be pierced and therefore the scale film on the plug is susceptible to wear and/or peeling off. Once the scale film on the plug has been worn out or peeled off, the heat-shielding effect diminishes and the temperature of the plug base metal rises, and the plug may possibly be damaged due to heat-related metal wastage.
When a scale film coated plug is used for piercing and rolling in the process of producing high alloy steel seamless pipes, the scale film is generally worn out in several rolling passes and, thus, the life thereof is very short.
If piercing and rolling is continued using a plug with a damaged scale film, seizure failure will occur due to the direct contact between the plug base metal and the billet surface and, at the same time, flaws will be developed on the inside surface of the hollow shell, impairing the quality of the product.
Therefore, in the production of high alloy steel seamless pipes, plug replacement is to be made frequently and, as a result, the production efficiency of the piercing and rolling mill declines. In particular, in the current situation in which efficient production of seamless steel pipes is aimed at and the continuous operation of the Mannesmann pipe making equipment reaches an advanced state, a reduction in production efficiency of the piercing and rolling mill exerts an influence on the efficiency of the whole process of production of seamless steel pipes.
For regenerating and reuse, the plug once used for piercing and rolling and demounted from the mandrel, it is necessary to re-form the scale film; however, lengthy time and a number of steps are required for the necessary heat treatment to that end. Accordingly, it is necessary to keep an immense number of plugs so that even when plug replacement is made with high frequency, any shortage of plugs may not occur to circumvent the decrease in production efficiency of the piercing and rolling mill.
In view of such situation, various investigations have so far been made to prolong the plug lifetime and thereby reduce, as far as possible, the number of plugs to be kept prepared. For example, Japanese Patent Publication No. 04-8498 (hereinafter referred to as “Document 1”) proposes a plug base metal increased in high-temperature strength by reducing the Cr content and adding Mo, W and/or the like as well as a plug with an oxide scale excellent in adhesiveness as being formed on and attached to the surface of the plug.
The plug proposed in Document 1 has a drawback in that when the billet piercing length is increased, the plug base metal becomes insufficient in both high-temperature strength and scale film adhesiveness, and hence a satisfactorily long lifetime cannot be secured.
Japanese Patent Application Publication. No. 04-74848 (hereinafter referred to as “Document 2”) and Japanese Patent Application Publication No. 04-270003 (hereinafter referred to as “Document 3”) propose plugs for which a plug base metal having a chemical composition suited for improvements in high-temperature deformation resistance and crack resistance is employed and on the surface of which an oxide scale is formed. However, the plugs proposed in these Documents 2 and 3 have a drawback in that the scale film in the plug tip portion, where the contact pressure is the highest and the temperature increases in piercing, is melted and loses the heat-shielding ability and wear resistance, with the result that the plug tip readily undergoes damages and/or deformation due to melting-related metal wastage.
Japanese Patent Publication No. 64-7147 (hereinafter referred to as “Document 4”) proposes a plug for which a plug base metal containing Cr and Cu as added thereto is used and on the surface of which a scale film is formed. However, the plug proposed in Document 4 also has a drawback in that the plug tip readily undergoes damages and/or deformation due to melting-related metal wastage on the occasion of piercing.
The plugs proposed in the above-cited Documents 1-4, the plug base metal chemical compositions of which have been adjusted, in any case, when applied to piercing and rolling in high alloy steel seamless pipe manufacture, cannot be expected to show satisfactorily prolonged lifetime through such plug base metal composition designing alone.
Further, the plugs proposed in the above Documents 1-4, when repeatedly used for piercing and rolling, all require a long period of heat treatment for scale film regeneration. Therefore, investigations have so far been made to increase the plug lifetime by changing a plug surface scale film with other than the oxide scale.
For example, Japanese Patent Application Publication No. 10-180315 (hereinafter referred to as “Document 5”) proposes a plug increased in high-temperature bending strength by partially replacing the plug surface with ceramic material such as SiC, Al2O3, ZrO2 or Si3N4 in lieu of the oxide scale. Japanese Patent Publication No. 59-13924 (hereinafter referred to as “Document 6”) proposes a plug with a film formed on the surface thereof by plasma spraying of an Fe oxide powder.
However, the plug proposed in Document 5 is susceptible to damaging of the ceramic portion due to the impact on the occasion of piercing and is thus required to be handled carefully on the occasion of plug replacement and handling. Furthermore, the ceramic-constituted plug itself is expensive and difficult to regenerate. Further, the plug proposed in Document 6 requires a large-scale apparatus for plasma spraying of powders for the preparation and regeneration of the plug, and hence requires huge costs.